Why

Usually because somebody asked for one file

An expense report, an insurance claim, a bundle of paperwork for an office — in all of them ten attachments come back with a request to combine them.

Expense reports

Receipts are photographed all month and handed in at once. One file in date order reads faster, and it is harder to lose an item in it.

Paperwork for an office

Here legibility matters, not weight. A photo taken at an angle or in shadow may be rejected even if it is clear to you.

Damage photos

Order tells the story: the whole scene first, then the close-ups. Once combined into one file, that order is fixed.